Abstract

The periodic number dependence of the femtosecond laser-induced crystallization threshold
of [Si(5nm)/Sb80Te20(5nm)]x nanocomposite multilayer films has been investigated by coherent phonon spectroscopy.
Coherent optical phonon spectra show that femtosecond laser-irradiated crystallization
threshold of the multilayer films relies obviously on the periodic number of the multilayer
films and decreases with the increasing periodic number. The mechanism of the periodic
number dependence is also studied. Possible mechanisms of reflectivity and thermal
conductivity losses as well as the effect of the glass substrate are ruled out, while
the remaining superlattice structure effect is ascribed to be responsible for the
periodic number dependence. The sheet resistance of multilayer films versus a lattice
temperature is measured and shows a similar periodic number dependence with one of
the laser irradiation crystallization power threshold. In addition, the periodic number
dependence of the crystallization temperature can be fitted well with an experiential
formula obtained by considering coupling exchange interactions between adjacent layers
in a superlattice. Those results provide us with the evidence to support our viewpoint.
Our results show that the periodic number of multilayer films may become another controllable
parameter in the design and parameter optimization of multilayer phase change films.